The present invention relates to a cone optical system wherein there is provided correction for the astigmatism of paraxial rays.
An optical system of this type can be used with particular interest in optical sensors for controlling the attitude of spinning artificial satellites in space. An optical sensor is a device installed aboard a satellite for sensing the light radiated by a celestial target (sun, moon, earth or a star) to convert it into an electric signal to be used thereafter to determine the deviation of the satellite axis relative to a reference direction.
Attitude sensors in spin stabilized satellites for coverage reasons frequently are required to have an extended field of view of up to 120.degree. or higher in one direction and field of view of about 1.degree. or less perpendicular to it. If the lengthwise extension of view (LEOV) of 120.degree. or higher is aligned approximately parallel to the spin axis, there is a high probability that the small crosswise extension of view (CEOV) crosses the reference target (sun, earth, star) and that the instant at which this occurs can be determined and used for attitude determination. Current designs of such sensors generally employ optical systems which collect the light coming from the target onto one or several slits behind each of which is a detector which transduces the optical signal passing through the slit into an electrical signal. This indicates the target crossing instant. An advanced embodiment for an optical sensor of this type is described in U.S. Pat. No. 3,838,277.
There are a number of disadvantages inherent to the known sensors, among others the following:
(1) the sensors should preferably have an elongated shape in order to collect the transmitted energy through the slit with the best efficiency as possible;
(2) THE IMAGE BRIGHTNESS DECREASES TOWARDS THE OUTER PORTIONS OF THE LEOV unless masks are implemented sacrificing the brightness in the central portion of the LEOV or unless use is made of complex optical systems which introduce distortion;
(3) THE IMAGE DEFINITION IN THE CEOV direction frequently deteriorates towards the edges of the LEOV;
(4) the extension of the LEOV beyond 120.degree. becomes increasingly hard and a limit is generally set by the image quality, the required detector length and the energy collection efficiency;
(5) SENSORS COMPRISING SEVERAL DETECTORS ARE PRONE TO ALIGNMENT PROBLEMS.
Another type of optical sensor comprises an optical system consisting of an inverted reflecting cone associated with a concentration lens as described in a paper entitled "Inside-Out Horizon Scanner" by J. Killpatrick, published in Applied Optics, Vol. 1, No. 2, March 1962.
This sensor, however, employs a reflecting cone, together with a collecting lens only without using a refracting cone. This results in high aberrations for paraxial rays. Moreover, the sensor is used in a radiation balance mode i.e. for non-spinning satellites, in such a way that its symmetry axis Z is pointing towards the target. In this mode, the sensor is suitable only for widely extended targets. For targets in a plane perpendicular to the optical axis, the optics as described by Killpatrick suffers from bad imaging properties.
The object of the invention is an optical system producing an image free of astigmatism for parallel rays and in which the imaging quality is independent from the azimuth of the radiation source.
When applied to an optical sensor for controlling the attitude of a spinning satellite, the invention has for object an optical sensor which permits substantially reducing the alignment problems and to utilizing detectors having circular shaped sensitive elements which are more easily available on the market.